Potentially a hydrogen rich gas can be used in a variety of internal combustion engine applications, especially on board gasoline and diesel internal combustion engine vehicles. For example, the availability of a hydrogen rich gas on board a vehicle equipped with a diesel engine could assist in reducing nitrogen oxides in diesel exhaust streams. Also, it could be used for removal of carbon from diesel particulate filters. Similarly, in gasoline vehicles a hydrogen rich gas could be used to extend the lean burn limit of the engine thereby improving the efficiency of the engine. It could be used for exhaust gas treatment. Those, of course, are just a few of the uses for a hydrogen rich gas on board a vehicle with an internal combustion engine. Additional uses will be described hereinafter.
Notwithstanding the potential use of hydrogen rich gas with internal combustion engine systems there is a need for practical methods for providing the hydrogen rich gas on board a vehicle.
Hydrogen may be produced from hydrocarbons in a fuel processor such as a steam reformer, a partial oxidation reactor or an auto-thermal reformer and a fuel cell system incorporating such hydrocarbon fuel processors has been proposed.
In the case of a steam reforming, steam is reacted with a hydrocarbon containing feed to produce a hydrogen-rich synthesis gas. The general stoichiometry, illustrated with methane, is:CH4+H2O→CO+3H2  (1)Typically an excess of steam is used to drive the equilibrium to the right. As applied to hydrogen manufacture, excess steam also serves to increase the water gas shift reaction:CO+H2O→CO2+H2  (2)
Because of the high endothermicity of the reaction, steam reforming is typically carried out in catalyst packed tubes positioned within a furnace that occupies a volume of space substantially greater than the tube volume. The large size of such conventional steam reformer is one factor that limits its use in space constrained applications such as on board vehicles.
Gas phase partial oxidation of hydrocarbons to produce hydrogen evolves involves feeding a hydrocarbon and sub-stoichiometric oxygen into a burner where they combust to produce a synthesis gas mixture. The ideal gas phase partial oxidation reaction illustrated for methane is:CH4+½O2→CO+2H2  (3)However, gas-phase reaction kinetics tend to over-oxidize some of the feed, resulting in excessive heat generation and substantial yield of H2O, CO2, unreacted hydrocarbons and soot. For these reasons when gas phase partial oxidation chemistry is applied to clean feeds, it is preferred to add steam to the feed and add a bed of steam reforming catalyst to the gas phase partial oxidation reactor vessel. This combination of gas phase partial oxidation and steam reforming is called autothermal reforming.
In autothermal reforming processes a source of oxygen such as compressed air is employed which results in a nitrogen-diluted synthesis gas that renders the gas less suitable for fuel cell use in space constrained applications.
Sederquist (U.S. Pat. Nos. 4,200,682, 4,240,805, 4,293,315, 4,642,272 and 4,816,353) discloses a steam reforming process in which the heat of reforming is provided within a catalyst bed by cycling between combustion and reforming stages of a cycle.
As described by Sederquist, the high quality of heat recovery within the reforming bed results in a theoretical efficiency of about 97%. However, these patents describe a process that operates at very low productivity, with space velocities of around 100 hr−1 (as C1-equivalent). Moreover, this process requires a compressor to compress the product synthesis gas to elevated pressure. One consequence of Sederquist's low space velocity is that resulting high heat losses impede the ability of this technology to achieve the theoretical high efficiency.
Kobayashi et al (U.S. Pat. No. 6,767,530 B2) discloses a steam reforming process in which a hot synthesis gas is produced in a heated regenerative reactor bed. The synthesis gas is then cooled and passed through an adsorber to adsorb synthesis gas species other than hydrogen. Thereafter the adsorbed species are desorbed and combusted with oxidant to produce a hot gas that is used to heat the regenerative reactor bed. Whatever the advantages of this process may be, there is no disclosure as to how to produce the high space velocities and high efficiency required for utilization in compact environments such as on board vehicles.
Hershkowitz et al (US 2003/0235529 A1), incorporated herein by reference, discloses a highly efficient and highly productive process for producing a hydrogen rich containing gas from a hydrocarbon containing fuel, the process being called “pressure swing reforming” or “PSR”.
PSR is a cyclic, two step process in which in a first reforming step a hydrocarbon containing feed along with steam is fed into the inlet of a first zone containing reforming catalyst. During the reforming step a temperature gradient across the reforming catalyst has a peak temperature that ranges from about 700° C. to 2000° C. Upon introduction of the reactants, the hydrocarbon is reformed into synthesis gas in the first zone. This reforming step may be performed at a relatively high pressure. The synthesis gas is then passed from the first zone to a second zone where the gas is cooled by transferring its heat to packing material in a second regeneration zone.
The second, regeneration step begins when a gas is introduced into the inlet of the second zone. This gas is heated by the stored heat of the packing material of the recuperation zone. Additionally, an oxygen-containing gas and fuel are combusted near the interface of the two zones, producing a hot flue gas that travels across the first zone, thus reheating that zone to a high enough temperature to reform feed. Once heat regeneration is completed, the cycle is completed and reforming may begin again.
PSR has a number of significant advantages over other synthesis gas processes. For example, the system is compact, making it especially suitable for space constrained applications. Also, the process can be operated at high space velocities making it especially efficient. And, PSR is capable of producing relatively high partial pressures of hydrogen compared to other reforming processes.
The practical application of any synthesis gas production technique or hydrocarbon conversion process will depend upon how well the upstream and downstream processing systems can be combined into an overall process design. The invention described below and defined in the claims provides a unique process design and system for generating a hydrogen rich gas and using the gas produced in an internal combustion engine system in a number of advantageous ways.